1 What is a star? A star is a giant ball of gases held together by gravity. It makes heat and light. Different stars produce different amounts of energy. The amount of energy given off determines the star's surface temperature and color. Red stars are cooler, yellow stars like our sun are a little hotter, and blue stars are the hottest.2 Stars are formed in nebulae. A nebula (plural nebulae) is a large cloud of dust and gas in space. The word nebula comes from the Latin word for cloud. Most of the gas in a nebula is hydrogen gas. Over billions of years, the cloud contracts and gets denser and denser as well as warmer and warmer. As more and more gas is pulled into the cloud, it begins to spin. As the cloud spins, atoms of hydrogen gas bump into one another. The faster the gas spins, the more the atoms bump together, and the temperature of the spinning cloud gets hotter.3 When the temperature reaches ten million degrees Celsius, a chemical change called nuclear fusion begins to take place. In this change, two atoms of hydrogen gas combine, or fuse together, to form an atom of helium gas. This chemical change gives off a large amount of energy in the form of heat. This is a process like the opposite of an atomic bomb. Just as splitting atoms (nuclear fission) gives off a huge amount of energy, the fusion that takes place in a star also releases huge amounts of energy. The result is the formation of a new star. The new star gives off heat and light from the nuclear fusion of hydrogen atoms. New stars are constantly forming in space.4 The star expands and "burns" brightly for a few billion years. Even though we use the word "burn" with stars, they are not actually on fire. The heat and light are released by the chemical process of atoms joining together. This middle stage in the life cycle of a star is called the main sequence. As the hydrogen is used up, the star begins to fuse helium and heavier elements. This uses more of its energy. The star begins to cool off, expand, and becomes a red giant. Red giants give off lots of light but not much energy. Smaller stars become white dwarfs as they get older, and large stars explode as supernovae which burn brightly and quickly. The material that explodes from a supernova becomes the gases and dust that creates new stars.